I always seem to have trouble against Zerg as Terran, but ESPECIALLY if there are a mass of early Mutas harrasing me. I know Marines are great against them but that's not good enough, because they aren't as mobile as the Mutas and unless that's their only unit I can't really just rely on the amount of Marines it would take as I need to build other units. However Vikings I've realized pretty much suck against Mutas as far as I can tell. I heard Thors are good but that's not reasonable for early game.

In the end your best strategy is not letting them get that far
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Ivo FlipseAug 15 '10 at 18:21

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I disagree with @Ivo. You really just don't have that much control over a match, and if you did you wouldn't need help countering Muta. Ultimately there is no magical unit which will guarantee victory vs a Muta heavy strategy. Instead, try to think of how to use what you have. I think the answers give a pretty good breakdown of the options.
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tzenesAug 16 '10 at 6:22

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@Ivo Flipse Any unit whose counter is "don't let the enemy get them" would be imbalanced almost by definition. mutas cost 100/100. marines cost 50 each. turrets also cost only minerals, and do serious dmg to mutas
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Peter RecoreAug 16 '10 at 15:28

@Peter I really dislike the term "unit counter," as no unit should grant instant victory against another unit type. Instead we should look for "strategy counters," where in one strategy provides benefit against another.
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tzenesAug 16 '10 at 17:45

@Ivo @Peter: Ivo is completely right in saying: Don't let the Zerg get that far. if a Zerg is teching for fast mutas, he will not be strong on the ground and you can crush him. If he is good he will harass you only with speedlings and try to keep you off his back. @Peter: Mutas are not counter-able except for Mass Vikings. And that is obviously only possible if you see it early enough. If there are already mutas picking of your expansions you will never get a big enough Viking fleet to counter the 8 mutas the zerg will push every spawn.
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TigraineAug 16 '10 at 23:03

4 Answers
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Obviously Missile Turrets just aren't good enough, as they're designed to stop harass (3-4 Muta) and not larger pushes. A large number of people will tell you Thors are the answer, and with their bonus to light and aoe this seems reasonable.

Let's take a moment to consider this. Mutalisk vs Thor.

First I'm going to need to pull in some numbers from a spreadsheet. We can see that Mutalisks get 8.15 dps and Thors get 16 dps vs light air. So already Thors have an advantage. Additionally, with splash, the Thor hits more than one Muta at a time. So this reasonable assumption seems to hold up very well.

Let us pretend we are Jane Goodhall for a moment and observe this match up in the wild. I happen to have a good match: Sen vs Demuslim (you can fast forward to 10 minutes in).

Strangely the Marine/Thor combo seems to die out to a pure Muta approach. Now part of this can be attributed to Sen's excellent play, but Demuslim is a very good player so that can't be the reason (where as Sen could stomp me with any unit). So why are thors dying?

Well Sen will attack with Muta ratios of 4:1 or 5:1, well beyond the cost ratio. Demuslim never seems to be able to field enough Thors. This is largely a result of the Thor's long build time (compared to Muta coming out of Hatcheries), and slow speed. Demuslim tries to compliment this with dropships, but those are more cost with no real combat benefit. What's more Demuslim is forced into making slow pushes against a very mobile Zerg force. By comparison, its very easy for Sen to build a lot of expansions and just get gas out of them, allowing for a more gas heavy force (remember gas is our limiting fact for building).

I won't ruin the end of the match, but at the very least we can see that Thors seem to die out to Mutalisks outside of the 30 vs 90 scenario.

So what's a Terran to do?

Well you can compliment your Thors with SCVs, but what you really need is more damage. Luckily enough there is a very cheap way for Terran to get more damage. Marines! Stimmed Marines are very effective vs Mutalisks. What's more they have no additional gas cost. What's more those Medivacs that you're carrying Thors around in really boost the survivability of those Marines. The best part is, there is no Terran strategy which doesn't allow for easy production of Marines. I'd also add that building Turrets is a very cheap way to slow down a Mutalisk attack and give your Marine/Thor forces time to get into position.

Finally, its worth noting that the first 2 points of armor do double work against Mutalisks as they reduce bounce damage as well.

I wrote this answer before Idra vs Tarson in IEM when Idra debut the "Magic Box." The Magic Box is a technique that Zerg can use to keep Mutalisks spread out as the engage Thors to minimize splash.

Its worth noting that all Zerg units are biological and thus the Ghost ability "Snipe" is effective against all of them. However, Ghost dps is fairly low (6.67) and only really improves vs light (13.33) and are actually more cost effective vs Mutalisks than Thors (though at lower hp). However, what really makes them work is that "Snipe." Snipe is worth 45 hp (goes through armor) for 25 energy, which means a full Ghost can Snipe 3.6 Mutalisks worth of damage (at only 1.5 the cost). What's more Ghosts have a faster build time (40s vs the Thor's 60s); are built out of Baracks (so synergize nicely with MMM play); can be healed by Medivac; and even go invisible to get the jump. Finally, when compared to Marines, Ghosts are NOT light (they have no armor type) and have 100 hp (twice the hit points), so aren't as susceptible to banelings (though are very weak against Muta into Roach).

great point on the stims, that would also give my marines a nice boost to actually make it over to where ever the mutas are attacking (obviously not the front of the base where the marines are usually camping)
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ioSamuraiAug 16 '10 at 12:45

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Careful with stim though, until you have medivacs any decent player will just back off and hit somewhere else while stim wears off. I've managed to get players to stim themselves down to red health chasing the mutalisks and then just crushed their marine army after it has no health left.
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en1gmatic325Aug 16 '10 at 17:36

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@en1gmatic is right, stim is only really effective with a medivac to back it up. Though having a medivac is a useful thing for Marine or Thor heavy play
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tzenesAug 16 '10 at 17:43

Stimmed marines are the answer to pretty much every light air unit, as well as void rays.
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Lotus NotesAug 16 '10 at 18:09

for the rest you should be building more Marines, since if he's able to get a lot of Mutalisks, you should be able to get a lot of Marines. You're probably focusing too much on Tech upgrading or building Factories and such.

Remember though! This strategy depends on your opponents play style and if he uses a different strategy than mass Mutalisks, you might be in trouble. Though Marines, Marauder and Medivacs is never a bad idea!

I would recommend sticking to marines and turrets at your mineral lines as an early defense, but if he is really going mass mutalisk switching to thors will be your best bet. Zerg's teching is actually the slowest in the game, so you can get thors around the same time as zerg can get mutalisks if you see it coming. At the very least you should be able to start getting them out before he has enough mutalisks to overcome your turrets and marines. If you are really caught off guard without enough air defenses though, you could always try a counterattack with what you have to force the mutalisks back home to defend while you get up some anti-air.

Going to go with "Don't let the opponent get them" but I will take it a step further than most care to explain.

A zerg going to early mutalisks will do a few things. Firstly, he will invest all/most of his gas into the Lair (100 gas) and spire (200 gas). Not only will he have used most of his gas on these things, he will be saving 100 gas per mutalisk he plans on building. If he manages to save about 700 gas (a LOT of gas especially early game), he will be able to morph 7 mutalisks as soon as the spire is complete.

Because he NEEDS to save so much gas to get a decent number of mutalisks early on, he will not have spent as much gas on his forces. We can assume that he has very few or no roaches/banelings, and unupgraded zerglings. Take advantage of this.

He is also incredibly vulnerable while his spire is morphing. Because:

he has had to save 200 minerals and 200 gas on this building

He is probably saving his larvae + injected larvae to morph into mutas. This is hugely important. This means that he is committed to not building any other units.

He is saving 100 minerals and 100 gas per muta he is planning on morphing

We can assume that he is indeed planning on morphing more than 2-3 mutalisks. This is because mutalisks are only effective in larger numbers.

But this is all useless if you don't even know if your opponent is going to build mutalisks. Practice scouting. Scout with a worker at 10 or 13 supply, don't be afraid of blowing a scanner sweep if you have no other options. Attack with your forces to see what his army is composed of, try to get into his base far enough to see things. Your first air unit is also incredibly useful as a scout.

The most important question you can ask yourself while playing (any game) is: "What is my opponent going to do?"